Stainless steel melting process



Patented Dec. 9, 1952 STAINLESS STEEL MELTING PROCESS Donald L. Loveless, Baltimore, MIL, assignor to Armco Steel Corporation, a corporation of Ohio No Drawing. Application October 27, 1950, Serial No. 192,598

Claims.

My invention relates to the production of corrosion and heat resisting steels, and more particularly concerns a method of producing stainless steels of a type responding consistently to extremely low carbon analysis.

Among the objects of my invention, therefore, is to provide a method of consistently and reliably producing stainless and other corrosion and heat resisting steels of extremely low carbon analysis which method is rapid, simple and economical in nature, which is certain and predictable in results, and which, moreover, adapts itself readily to the use of a wide variety of sources of iron and chromium which are readily available and of low cost, employing known and readily available tried and proved furnacing and operating equipment.

Other objects of my invention, together with many important and highly practical advantages thereof, will in part be obvious and in part pointed out hereinafter during the course of the following description.

My invention accordingly may be seen to reside in the several operational, procedural and manipulative steps, as well as in the relation and combination of each of the same with one or more of the others, the scope of the application of all of which is more fully set forth in the claims at the end of this specification.

As conducive to a more thorough understanding of certain important features of my invention it may be noted at this point that where I refer to the production of corrosion and heat resisting steels I have reference illustratively to steels possessing substantial chromium content. Typically these may contain approximately 4% to 30% or more chromium with the nickel content ranging up to about 30% or more, with the remainder substantially all iron. It is of course contemplated that desired alloying additions will be employed in small amounts for specialized purposes. Among these are included, illustratively, sulphur, selenium, phosphorus, copper, molybdenum, tungsten, tantalum, columbium, titanium, vanadium, silicon, manganese, aluminum and the like.

It is further to be noted that while in certain specialized instances the carbon content of such corrosion and heat resisting steels is intentionally high, usually it is desirable and even essential that the carbon content be kept extremely low and it is well recognized in the art that much effort has been directed towards the achievement of this result.

Accordingly, my invention is directed to ameth- 2 0d of consistently producing low carbon stainless and. other corrosion and heat resisting steels, particularly where a carbon content cannot be tolerated in excess of about 0.03%.

It is an incidental feature of great practical importance in my new practice that effective and advantageous reclamation and use of metal scrap is achieved, particularly stainless steel metal scrap as is found about the melt shop, the rolling mill, and in customer plants. Since reclamation of such scrap metal, however, does not in itself comprise an essential feature of my present invention, no emphasis will be placed in this disclosure upon the effective use of scrap metal other than what becomes necessarily and inevitably attendant upon full and proper disclosure of my invention. Suifice it to say, then, that from a practical and economical standpoint, I employ a balanced furnace charge comprising stainless steel scrap and chromium ore, two radically divergent sources of chrome metal and both of which exist in abundant quantity, each being available at comparatively low cost.

I have recalled at an earlier point herein that stainless and other corrosion and heat resisting steels are demanded responding to a low carbon analysis. It is in the very obtention of this low carbon content, however, that considerable practical diificulty is observed, not so much in bringing the carbon content to desired low value in the initial or oxidizing phase of the melt, but in retaining the low carbon content during the second or reducing phase of the melt, during the course of which the iron and chrome content is recovered from the slag and recaptured in the underlying pool of molten metal. In previous practices, where there is employed an initial melt-down or oxidizing period followed by a second or reducing period, it has been customary at the beginning of the reducing period to charge a mixture of lime and crushed ferrosilicon onto the very refractory crusty slag which prevails at the end of the first or oxidizing stage. Power is then applied to the furnace and the electrodes lower themselves until an arc is established. This are is not established, however, until the electrodes have actually come in contact with, or almost in contact with, the metal bath.

If contact is actually established betweenelectrodes and the metal bath carbon, absorption occurs. And even when contact is not quite established the force of the arc throws away the crusty poorly conducting slag to such an extent that the arc is established between the electrode and the metal for a certain period of time, thereby permitting absorption of the carbon vapor of the intense are. As the operation continues the surrounding line and ferrosilicon becomes molten around the electrodes and they gradually rise upward as the depth of the slag layer increases. Inevitably relatively cold lime and ferrosilicon fall down into the arc area from time to time causing are extinction and subsequent automatic lowering of the electrodes to reestablish the arc. Melting in the electrode area proceeds from the surface of the bath upward until the lime and ferrosilicon become incorporated in the molten slag. Some carbon contamination from the electrodes therefore seems inevitable.

I have found in my investigations looking toward the production of stainless steel of extremely low carbon content that additional carbon contamination seems to come from the lime employed in the reducing period. Apparently the lime as it ordinarily is available for meltshop practice has picked up CO2 from the air while the lime is standing around the plant prior to use. Or possibly the lime has not been completely burned in the first place. In any event, some CO2 normally is present in the lime as calcium carbonate and when the lime is charged into the furnace I find that this CO2 is released. The CO2 reacts with the molten stainless steel in such a way as to favor carbon absorption by the steel.

Thus, while it is possible to achieve a desired low carbon content at the conclusion of the initial or oxidizing stage, and in point of fact the low carbon analysis frequently is employed as a criterion empirically indicating the conclusion of the oxidizing stage, difiiculty is thereafter experienced in holding the carbon content of the metal to this desired low value during the subsequent reducing stage. This heretofore has been well nigh imposible in many instances and the increase in carbon has been unpredictable and extremely diflicult of control.

An important object of my invention, therefore, is to provide a method, essentially economical and practical in nature and attended by consistent and predictable results, for the production of stainless steels responding closely to required extra low carbon analysis.

In accordance with the practice of my present process I produce stainless steel of extra low carbon content by minimizing carbon contamination from the lime employed during the reducing stage of the process and from the furnace electrodes. I find that to remove effectively the CO2 content from the lime it is not enough simply to pre-heat the lime; for by so doing the CO2 is not driven off at the temperatures ordinarily employed for removing moisture from the lime. Thus, at the conclusion of the ordinary preheating treatment of the lime, while the moisture will have been effectively removed, thereby effectively eliminating hydrogen contamination of the metal and the porosity and gasiness attendant thereupon, quite on the contrary the carbon contamination of the metal has not been avoided, and the resulting metal displays a detrimentally high carbon analysis.

I find that the lime which is employed as a flux preliminarily must be exposed to a relatively high temperature, say a minimum of about 1495 F. At this temperature all'the CO2 apparcntly is driven off, and none remains in the lime to subsequently come out and contaminate the metal with carbon. Accordingly, in a typical practice of my invention I charge onto the slag, during the initiation of the reducing stage of the process while the electrodes are raised, lime which has been subjected to an adequately high temperature, say in excess of 1495 F. and as a result of which substantially all CO2 content thereof has been expelled. The furnace electrodes then are lowered and arcs established on the ferrosilicon. Thus by highly heating the burned lime prior to charging it into the furnace and by charging the ferrosilicon reducing agent onto this lime and arcing the furnace electrodes onto the ferrosilicon I have obtained melts responding to a carbon analysis consistently below 0.030%, sample heats illustratively displaying carbon contents of 0.017%, 0.018%, etc.

As illustrative of the practice of my invention, wherein a low carbon 18-8 stainless steel is sought, employing say a sixteen-ton Heroult furnace with furnace banks formed of chromite brick, I employ a balanced charge of steel scrap comprising, in varying quantities ordinary steel scrap, 18-8 chromium-nickel stainless steel scrap, and chrome ore, along with small percentages of electrolytic nickel, high carbon ferrochrome and iron ore.

In this typical embodiment, I first charge substantial quantities of chrome ore onto the hot banks of the furnace (about 3000 F.) to dry the same and to-remove moisture therefrom. This being laid down to a thickness of say ten inches. Next, the scrap metal, representing typically the scrap available in the melt shop and in the customer plants, is charged onto the bottom of the furnace. I then charge the iron ore on top of the scrap. Where there is likelihood of excess moisture content sufficient to require pre-drying and pre-heating of the iron ore according to prior furnace practice, then I also charge this iron ore onto the banks of the furnace as in the case of the chrome ore. This mode of charging the ingredients of the heat forms the subject matter of my copending application Serial No. 181,565 of August 25, 1950, entitled Stainless Steel Melting Process. And while the typical embodiment herein described contemplates charging the chrome ore onto the banks of the furnace, I do not desire to be restricted to only this mode of introducing the component materials into the furnace. For illustratively, it is entirely within the contemplation of my invention to remove moisture from the chrome ore, and iron ore as well, by pre-drying in accordance with the existing practice.

Preferably I also employ a small quantity of fluorspar, during the early part of the initial or oxidizing stage of the process, to flux the chrome ore overlying the banks of the furnace. According to this practice, I charge 300 pounds each of fluorspar and freshly burned lime onto the chrome ore. Here again the thickness of the layer is such that the ingredients are brought to required high temperature before reaching the bath itself.

The chrome ore charged onto the banks of the furnace amounts to about 400 pounds per ton of steel to be produced. Any chrome ore in excess of the above amount is charged directly on top of the scrap in the furnace without introduction onto the furnace banks. Moreover, it is to be noted that chrome ore which is to be charged directly onto the scrap is charged after the heat has been started, so that if any chrome ore sifts through the scrap, it will go into the slag where it is heated sufiiciently to eliminate moisture.

In the practice of my invention it will be found that after about 1 hours, the scrap has melted down forming a bath of molten metal, and the chrome ore has melted to form a slag overlying the bath. The melt-down or oxidizing period is carried to a point where the carbon content has been brought to the desired value of less than 0.030%. And, I find that the oxidation of the carbon is greatly accelerated by employing an elevated temperature, say of the order of about 3100 F. to 3250 F., constituting a value of some 150 to 200 F. greater than the temperatures ordinarily employed in steel making practices. I conveniently call this a temperature of superheat. As soon as this low carbon content has been achieved the oxidizing stage is completed.

As previously noted, a substantial quantity of chromium, as chromium oxide, is carried over into the slag during the oxidizing stage. There also is present in the slag the large quantities of chromium oxide contained in the chrome ore. For efficient melt shop practice this chromium must be recovered during the subsequent reducing stage along with the iron likewise present in the slag as an oxide. To this end, upon concluding the melt-down or oxidizing period, and generally following customary furnacing practice, I employ a substantial quantity of non-carbonaceous reducing agent, of which ferrosilicon is typical. Usually I introduce this in an amount chemically in excess of the oxides of iron and chromium contained in the slag. And, this ferrosilicon is charged onto the furnace slag in the manner hereinafter defined. It remains, during the reducing stage of the process, wherein the chrome and iron are recovered from their oxides as contained in the slag, to maintain the carbon content of the metal bath at the low figure theretofore achieved. There exists, however, extreme hazard of carbon contamination during this reducing period because the furnace conditions are non-oxidizing with respect to carbon and any carbon taken up is retained rather than elimi nated. Accordingly, all ingredients charged into the furnace during this stage are maintained at minimum carbon content in an eifort to regulate this factor within close limits.

In furtherance of the practice of my invention then, I raise the furnace electrodes and charge lime in pebble form on top of the slag to form a thick blanket overlying the slag. This lime, as previously noted, has been preliminarily heated to a temperature of at least 1495 F. in order to drive off all C02 present including even that which might be taken up from the atmosphere in standing. Of necessity, all moisture present in the lime is driven off ahead of the CO2 during the preliminary heating.

Ferrosilicon is then charged separately in the area roughly defined by a circle about the three electrodes. This ferrosilicon remains on top of the blanket of lime and the power is applied and the electrodes are allowed to lower themselves. The initial contact is made with the electrically conductive metallic ferroscilicon lying on top of the hot lime. This contact is suiiiciently good to cause the establishment of the three arcs at or near the surface of the layers of lime and ferrosilicon. The ferrosilicon melts, thereby rapidly fluidifying the slag under the electrodes and increasing the electrical conductivity so that the arcs are continually maintained. Incidentally, I find that the exothermic reaction between ferrosilicon and the reducible oxides in the slag facllitate the melting of further ferrosilicon and lime.

As the oxidized silicon (silica) fluxes the lime the liquid pools become larger and larger until the entire bath is liquefied without the electrodes making contact with or approaching closely to the metal bath, thus avoiding carbon contamination. This new procedure, it is to be observed, involves melting downward from the top of the material charged at the initiation of the reducing period toward the metal bath, instead of upward from at or close to surface of the metal bath toward the upper slag surface as in the prior practice.

Thus the full benefits of the reducing stage are achieved and steel of extremely low carbon content is assured. Moreover, the slag is fluid and workable, and a generally improved metal is produced. The lime effectively maintains assurance of a basic slag during the reducing stage, effectively preventing silicon contamination of the metal despite the presence of the required chemical excess of silicon.

Illustratively for a heat of say roughly 15 tons of metal, following the raising of the furnace electrodes, I spread about 8200 pounds of pebble lime over the slag at the beginning of the reducing stage, and then charge about 2400 pounds of 75% ferrosilicon on top of this lime. I have found it to be entirely satisfactory to charge into the slag about 5 boxes of lime and then in the area under the electrodes a box of ferrosilicon.

During the reducing stage of the process the lime preferably is kept above a red heat, illustratively, a cherry red ranging between 1600 F.- 1800 F. The amount of lime immediately beneath the electrodes is kept as small as possible. The ferrosilicon employed is preliminarily screened to eliminate carbonaceous impurities as is true of all other additives employed during the reducing stage. This ferrosilicon is scattered over the lime largely within the area immediately beneath the furnace electrodes as previously noted. The electrodes are then lowered, the power applied and the reducing period carried forward.

After all of the ferrosilicon and lime have fused and completed their action with the ingredients contained in the slag and metal, and a substantially complete recovery of the oxides of both the iron and chromium of the slag is achieved, as evidenced by the color of successive samples of the slag taken from the furnace changing from a black color to a light gray or green, then the reducing period is at an end. High recovery is had with substantially no carbon contamination. The carbon content is maintained during the reducing stage within the order of that had at the end of the melt-down or oxidizing stage. Perhaps the carbon of the metal bath continues to be oxidized beneath the oxidizing slag, passing off into the atmosphere as CO2, this continuing until the silicon coming from the melting ferrosilicon percolates through the lime and enters the metal bath. This would have the effect of minimizing the carbon content of the metal bath and keeping it at a minimum for as long as possible.

Experience has disclosed that 42 out of 44 heats made in accordance with the practice of my invention, display carbon content in the finished steel below 0.030%, with some heats being even under 0.020%. And this is true even though the ferrosilicon employed may contai'n'as much as 0.050% carbon.

Illustratively, the carbon analysis of eight successive heats of the 44- made according to the practice of my invention is as follows:

C arbon content in percent Grade of Metal 20-14 MO .i

In every instance, the carbon content is substantially below 0.030%. Each of the 44 heats produced some 13 to 14 tons of metal.

Thus it will be seen that there is provided in my invention a corrosion and heat resisting steel melting process in which the various objects hereinbefore recited are successfully achieved. The finished product is found to adhere closely to desired chromium and nickel contents, while and iron contents of the slag. All these as well as many other highly practical advantages attend upon the practice of my invention.

It is apparent that once the broad aspects of my invention have been disclosed, many embodiments will suggest themselves to those skilled in the art to which the invention relates. Moreover, many modifications of the present embodiment will be advanced, all falling within the scope of my invention. Accordingly, I desire the foregoing description to be considered simply as illustrative, and that it not be construed as a limitation.

I claim as my invention:

1. In the production of corrosion resisting and heat resisting steel having a carbon content not exceeding 0.03% in an electric arc furnace, the art which includes melting metal scrap along with chrome ore until the charge is completely melted down and the carbon content of the bath has reached a desired low value; charging onto the slag burnt lime which has been preliminarily heated to a temperature and for a time sufficient to expel all carbon dioxide therefrom; charging crushed silicon-containing reducing agent onto the lime within said furnace; and arcing the furnace electrodes onto said reducing agent, whereby the iron and chrome content is recovered from oxidesv in the slag with minimum. carbon contamination.

2. In the production of corrosion resisting and heat resisting steel having a carbon content not exceeding 0.03% in an electric arc furnace, the art which includes melting down a charge comprising base steel scrap, stainless steel scrap,

ferro-chrome of substantial carbon content,

chrome ore and iron oxide, thereby forming a bath of ferrous metal containing 'carbon and chromium covered by a slag rich in iron oxide and chromium oxide; then after said charge is completely melted down and the carbon content of said bath has reached a desired low value, charging lime directly onto the slag which lime preliminarily has been heated to a temperature of at least 1495 F. to eliminate carbon dioxide; charging a crushed silicon reducing agent onto the lime; and thereafter maintaining the arc between the furnace electrodes across said reducing agent, the oxides of iron and chromium in the slag being thereby reduced and enriching the bath in the resulting metals in the substantial absence of carbon contamination.

3. In the production of corrosion resisting and heat resisting steel having a carbon content not exceeding 0.03% in an electric arc furnace, the art which includes melting down a charge of ingredients, including chrome ore, the chrome ore being charged onto the hot banks of the furnace in a layer of such thickness that substantially all moisture is driven from the ore before it enters the melt; and after complete melt-down of the charge to form a bath of metal with an overlying slag, and the carbon has reached a desired low value, charging onto the slag lime burned at such a temperature and for such a length of time as to expel all carbon dioxide therefrom; then charging crushed ferrosilicon onto the lime; and arcing the furnace electrodes onto the ferrosilicon, whereby the iron and chrome oxides contained in the slag are reduced with minimum carbon contamination of the metal.

4. In the production of corrosion resisting and heat resisting steel having a carbon content not exceeding 0.03% in an electric arc furnace wherein there first is formed a bath of ferrous metal with an overlying slag containing the oxides of iron and chromium the step, in the reducing stage of the process, of charging lime on top of the slag, all carbon dioxide present in the lime having first been driven off by heating the lime to high tem perature; then charging a layer of crushed lowcarbon ferrosilicon in chemical excess on the blanket of lime thus provided; and establishing an arc between the furnace electrodes and the layer of ferrosilicon.

5. In the production of stainless steel having a carbon content not exceeding 0.03% in an electric arc furnace containing a bath of ferrous metal of carbon content not exceeding 0.03% and an overlying slag containing the oxides of iron and chromium, the step which comprises introducing onto the slag of the melt during the reducing stage pebble lime from which all carbon dioxide has first been removed by heating the lime to a temperature of at least 1495" F., and charging crushed ferrosilicon onto the lime.

DONALD L. LOVELESS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,715,979 Bigge et al June 4, 1929 1,932,252 Arness Oct. 24, 1933 2,455,074 Loveless Nov. 30, 1948 2,465,383 Malcolm Mar. '29, 1949 

1. IN THE PRODUCTION OF CORROSION RESISTING AND HEAT RESISTING STEEL HAVING A CARBON CONTENT NOT EXCEDING 0.03% IN AN ELECTRIC ARC FURNACE, THE ART WHICH INCLUDES MELTING METAL SCRAP ALONG WITH CHROME ORE UNTIL THE CHARGE IS COMPLETELY MELTED DOWN AND THE CARBON CONTENT OF THE BATH HAS REACHED A DESIRED LOW VALUE; CHARGING ONTO THE SLAG BURNT LIME WHICH HAS BEEN PRELIMINARILY HEATED TO A TEMPERATURE AND FOR A TIME SUFFICIENT TO EXPEL ALL CARBON DIOXIDE THEREFROM; CHARGING CRUSHED SILICON-CONTAINING REDUCING AGENT ONTO THE LIME WITHIN SAID FURNACE; AND ARCHING THE FURNACE ELECTRODES ONTO SAID REDUCING AGENT, WHEREBY THE IRON AND CHROME CONTENT IS RECOVERED FROM OXIDES IN THE SLAG WITH MINIMUM CARBON CONTAMINATION. 